A disk drive is a data storage device that stores data in concentric tracks on a disk. Data are written to or read from the disk by spinning the disk about a central axis while positioning a transducer near a target track of the disk. During a read operation, data are transferred from the target track to an attached host through the transducer. During a write operation, data are transferred in the opposite direction.
During typical disk drive operation, the transducer does not contact the surface of the disk. Instead, the transducer rides on a cushion of air generated by the motion of the disk. The transducer is normally mounted within a slider structure that provides the necessary lift in response to the air currents generated by the disk. The distance between the transducer and the disk surface during disk drive operation is known as the “fly-height.”
The fly-height is controlled by a suspension attached to the slider. The suspension supports the read/write elements (i.e., transducer) and the air bearing of the slider. Certain conditions can alter the fly-height by creating disturbances between the airbearing and the disk surface. These conditions generally include extremes in environmental conditions such as the ambient air pressure (typically a function of altitude) and temperature, as well as extremes in contamination levels. Extremes in any of these conditions are taken into account during the development of airbearing designs because they may cause degradations in the error rate performance of the drive.
Because the transducer is held aloft during disk drive operation, friction and wear problems associated with contact between the transducer and the disk surface are therefore usually avoided. However, due to the extremely close spacing of the heads and disk surface, errors in fly-height control can lead to a head crash, where the head scrapes on the platter surface and grinds the thin magnetic film away. For giant magnetoresistive head technologies (GMR heads) in particular, a minor head contact that does not remove the magnetic surface of the disk can still result in the head temporarily overheating due to friction with the disk surface, rendering the disk unreadable until the head temperature stabilizes.
Dynamic fly-height control technologies have therefore become essential in many recent hard drive recording products. The technology is still in its early stages of use and adoption, however, and typical usage models have been relatively simple implementations. In one example, the fly-height is adjusted dynamically according to an average behavior of a population of hard drives operating under normal environmental conditions. However, this approach does not provide adequate performance when the hard drive is operated at environmental extremes. Deviations of individual read/write head behaviors from the population average behavior may be so large that control actions that would be effective for the average population instead result in over- or under-controlling the fly-height. These excursions of the control actions eventually cause reliability problems when many hard drives are deployed in the field.
Therefore, what is needed is a method of controlling the fly-height that effectively accounts for the individual behaviors of substantially all hard drives under extreme environmental operating conditions.